Dally is not essential for Dpp spreading or internalization but for Dpp stability by antagonizing Tkv-mediated Dpp internalization

Abstract

Dpp/BMP acts as a morphogen to provide positional information in the Drosophila wing disc. Key cell-surface molecules to control Dpp morphogen gradient and signaling are heparan sulfate proteoglycans (HSPGs). In the wing discs, two HSPGs, the glypicans Division abnormally delayed (Dally) and Dally-like (Dlp) have been suggested to act redundantly to control these processes through direct interaction of their heparan sulfate (HS) chains with Dpp. Based on this assumption, a number of models on how glypicans control Dpp gradient formation and signaling have been proposed, including facilitating or hindering Dpp spreading, stabilizing Dpp on the cell surface, or recycling Dpp. Here we generate genome-engineering platforms for the two HSPGs and find that only Dally is critical for Dpp gradient formation and signaling through interaction of its core protein with Dpp. However, we find that this interaction is not sufficient and the HS chains of Dally are essential for Dpp gradient formation and signaling, but surprisingly, largely without interacting with Dpp. We provide evidence that the HS chains of Dally are not essential for spreading or recycling of Dpp but for stabilizing Dpp on the cell surface by antagonizing Dpp internalization through Tkv. These results provide new insights into how distinct HSPGs control morphogen gradient formation and signaling during development.

Shaping and interpretation of Dpp morphogen gradient by endocytic trafficking

“…Unlike the GFP-dpp allele, the two newly generated alleles were not haploinsufficient but semi-lethal. To overcome the partial embryonic lethality, we introduced a transgene called “JAX”, which contains the genomic region of dpp critical for the early embryogenesis 29 but does not rescue the wing phenotypes of dpp mutants 20 . We found that JAX greatly rescued the lethality of each allele and each allele becomes easily homozygous viable without obvious phenotypes (Fig.…”

“…Endocytic trafficking has been proposed to regulate extracellular Dpp gradient formation through transcytosis 16 , recycling 17 , or a sink 20-22 . We found that extracellular Dpp distribution expands upon blocking early endocytosis but is not largely affected upon blocking the following endocytic trafficking such as MVBs formation, late endosome, or recycling, supporting that early endocytosis simply acts as a sink for Dpp.…”

“…In this case, Dpp is thought to bind to Tkv intracellularly to activate Dpp signaling. Although both models have been challenged 18-20 , endocytic trafficking may control Dpp spreading through other cell surface factors. Third, Tkv-mediated endocytosis has been proposed to simply acts as a sink to remove extracellular Dpp 20-22 , and Dally antagonizes this process to establish a long range Dpp gradient 20,21 .…”

“…Although both models have been challenged 18-20 , endocytic trafficking may control Dpp spreading through other cell surface factors. Third, Tkv-mediated endocytosis has been proposed to simply acts as a sink to remove extracellular Dpp 20-22 , and Dally antagonizes this process to establish a long range Dpp gradient 20,21 . Thus, regardless of the role of extracellular and cell-surface factors on regulation of the extracellular Dpp gradient, if and how endocytic trafficking itself influences the extracellular Dpp gradient remains unclear.…”

Stemming Tumoral Growth: A Matter of Grotesque Organogenesis

“…That is, the Dpp gradient scales with tissue size [ 116 , 120 ]. To fulfil this job, there is a mechanism which tunes the during organ growth, keeping the gradient and tissue proportional [ 87 , 88 , 116 , 125 , 126 , 127 ]. In this particular organ (i.e., wing), Dpp signalling scaling failure leads to defects in tissue size and pattern formation [ 87 , 88 , 116 , 126 , 128 , 129 ].…”